<?php
session_start ();
$num1 = isset ( $_POST ['num1'] ) ? $_POST ['num1'] : null;
$_SESSION ['num1'] = $num1;
?>
<head>
<meta charset="utf-8">
</head>
<body>
<h2>Introduce otro número</h2><br/>
<form action="fin.php" method="post">
<input type="hidden" name="num1" value="$num1">
<input type="text" name="num2">
<input type="submit" value="Siguiente">
</form>
</body>